98 Prof. Challis on a Theory of Magnetic Force. 
of the mutual action between two magnets (art. 7). As the 
two kinds of force act along the same lines in the same direc- 
tions, the consideration of this second force will not alter the 
conclusion arrived at in art. 4, as, in fact, is stated at the 
conclusion of that article. With respect to the manner in 
which the mutual action of two magnets is affected by the 
motion of the xtherial streams within them, it is to be remarked 
that divergence and convergence are increased by the opposi- 
tion of two streams, and diminished by their confluence; and 
that the particular force under consideration is always towards 
positions from which the streams diverge, or towards which they 
converge. Hence this additional force, being some function 
of the degree of convergency or divergency, produces effects 
exactly like those of the pressures considered in art. 7, and the 
conclusion there drawn remains the same when both kinds of 
force are taken into account. 
25. The following theory of magnetic induction rests on the 
foregomg views. It is found that when a bar of soft iron is 
placed in the plane of the magnetic meridian, it is magnetized 
inductively, the magnetism disappearing when its axis is placed 
perpendicular to that plane. ‘The induced magnetism is greatest 
when the axis coincides with the direction of the dippmg-needle ; 
and the magnetism of its north end is the same as that of the 
north end of the needle. These facts admit of beg explained 
on the supposition that the primary terrestrial stream in its 
passage through the bar exerts a force on the atoms in the di- 
rection from south to north, so as to produce a gradation of 
density of the bar towards the north end. ‘This force may ori- 
ginate in the circumstance that when the stream enters the 
bar its velocity is increased by the contraction of the channel by 
the atoms, and, the density and pressure bemg consequently 
diminished, the surrounding fluid is drawn towards the axis of 
the stream. ‘Thus there will be lines of motion without and 
within the bar converging northward, and consequently, from 
what is argued in art. 20, a force will be generated proper for 
producing the increment of density towards the north end. The 
same effect would be produced even if the streams were parallel, 
because as divergent streams fiowmg through iron, draw the 
atoms towards the quarter from which they flow, @ fortiori, 
parallel streams would do the same. Thus the bar is converted 
intoa magnet. But itis to be observed that the atoms retain their 
positions m consequence of the counteraction of this disturbing 
force by the molecular repulsion of the iron due to the variation 
of its density. Hence when the bar, by being put intoa position 
transverse to the magnetic meridian, ceases to be under the in- 
fluence of the terrestrial current, the atoms return to their normal 
positions, and the bar ceases to be a magnet. It is clear that this 
